The present application relates to a band-pass filter device, a method of manufacturing the same, a television tuner, and a television receiver.
Description will be made of a current system of a television (hereinafter described as TV) channel tuner.
A frequency selecting filter system realized by varying a coil (L) and a capacitor (C) as external parts is now used most widely as a method for selecting a desired channel from a wide frequency range (fractional bandwidth (%) varies greatly in a wide range of a frequency band in a case of a uniform bandwidth in particular) as in a tuner of a high-quality terrestrial television receiver.
A frequency selecting filter of a tuner of a TV receiver or a radio receiver is thus formed by a resonant circuit having a coil and a capacitor connected to each other.
The resonant circuit is formed by connecting the coil (L) and the capacitor (C) in parallel with each other, and the resonance frequency of the resonant circuit is given by an equation f=1/(2π√{square root over (()}LC)), where the unit of L is H (henry), and the unit of C is F (farad).
A TV receiver in particular selects a channel in wide frequency ranges of a VHF band and a UHF band. Thus, an optimum coil (inductance) needs to be selected for each frequency range, and a large number of external coils are often used. This is because when a resonant circuit is formed using a same coil for all of the wide frequency ranges, the pass band width of the channel filter varies greatly in each frequency range.
Present techniques for forming an on-chip filter on a wafer will next be described in the following.
There are techniques that do not use external parts such as a coil or a capacitor as described above or the like at all and which are regarded as promising for achieving excellent characteristics. The techniques include a band-pass filter using a “micro electromechanical system (MEMS) resonator element” or a “thin-film piezoelectric resonator element” that can be manufactured on a wafer by a semiconductor manufacturing process. Progress has been made in development of a system in which all channels are arranged on a chip using these resonator elements formed on the chip and selected by a switch in place of the tunable system of a current tuner.
For example, the concept of a piezoelectric resonator element device in which a plurality of piezoelectric resonator element groups forming a filter and having different frequencies are laminated so that desired resonator element groups are formed integrally has already been disclosed (see Japanese Patent Laid-Open No. Sho 55-50720, for example).
However, this technique laminates a plurality of resonator elements, and thus does not form a plurality of resonator elements en bloc on a wafer. Thus, the number of processes is increased, and the processes become complex, which raises a fear of an increase in manufacturing cost. The technique is therefore impractical.
Another technique for realizing a variable filter by applying a series voltage to a resonator element forming a filter and thus changing resonance frequency has been disclosed (see Japanese Patent Laid-Open No. 2003-168955, for example).
However, a range of variation of the resonance frequency is very limited (about a few %), and thus the technique cannot handle an entire frequency range of actual TV channels (from a VHF band (174 to 240 MHz) to a UHF band (470 to 862 MHz)).
Meanwhile, many examples of a resonator element and a filter formed onto a wafer have been disclosed (see Non-Patent Documents 1 to 16 listed below).
[Non-Patent Document 1]
“Single-Chip Multiple-Frequency ALN MEMS Filters Based on Contour-Mode Piezoelectric Resonators” 2007/4/Microelectromechanical Systems, Journal of Volume 16, Issue 2, pp. 319-328, April 2007
[Non-Patent Document 2]
“Piezoelectric Aluminum Nitride Vibrating Contour-Mode MEMS Resonators” 2006/12/Microelectromechanical Systems, Journal of Volume 15, Issue 6, pp. 1406-1418, December 2006
[Non-Patent Document 3]
“Aluminum Nitride Contour-Mode Vibrating RF MEMS” 2006/6/Microwave Symposium Digest, 2006. IEEE MTT-S International, pp. 664-667, June 2006
[Non-Patent Document 4]
“Behavioral Modeling of RF-MEMS Disk Resonator” 2006/12/MEMS, NANO and Smart Systems, The 2006 International Conference on Dec. 2006, pp. 23-26, December 2006
[Non-Patent Document 5]
“Mechanically Coupled Contour Mode Piezoelectric Aluminum Nitride MEMS Filters” 2006/1/Micro Electro Mechanical Systems, 2006. MEMS 2006 Istanbul. 19th IEEE International Conference on 2006, pp. 906-909, 2006
[Non-Patent Document 6]
“One and Two Port Piezoelectric Contour-Mode MEMS Resonators for Frequency Synthesis” 2006/9/Solid-State Device Research Conference, 2006. ESSDERC 2006. Proceeding of the 36th European, pp. 182-185, September 2006
[Non-Patent Document 7]
“AlN Contour-Mode Vibrating RF MEMS for Next Generation Wireless Communications” 2006/9/Solid-State Circuits Conference, 2006. ESSCIRC 2006. Proceedings of the 32nd European, pp. 62-65, September 2006
[Non-Patent Document 8]
“PS-4 GHZ Contour Extensional Mode Aluminum Nitride MEMS Resonators” 2006/10/Ultrasonics Symposium, 2006. IEEE, pp. 2401-2404, October 2006
[Non-Patent Document 9]
“AlN Contour-Mode Vibrating RF MEMS for Next Generation Wireless Communications” 2006/9/Solid-State. Device Research Conference, 2006. ESSDERC 2006. Proceeding of the 36th European, pp. 61-64, September 2006
[Non-Patent Document 10]
“High-Q UHF micromechanical radial-contour mode disk resonators” 2005/12/Microelectromechanical Systems, Journal of Volume 14, Issue 6, Dec. 2005, pp. 1298-1310, December 2005
[Non-Patent Document 11]
“Low motional resistance ring-shaped contour-mode aluminum nitride piezoelectric micromechanical resonators for UHF applications” 2005/1 Micro Electro Mechanical Systems, 2005. MEMS 2005. 18th IEEE International Conference on 30 Jan.-3 Feb. 2005, pp. 20-23, 2005
[Non-Patent Document 12]
“Finite Element-Based Analysis of Single-Crystal Si Contour-Mode Electromechanical RF Resonators” 2004/8/MEMS, NANO and Smart Systems, 2004. ICMEMS 2004. Proceedings. 2004 International Conference on 25-27 Aug. 2004, pp. 461-465, 2004
[Non-Patent Document 13]
“Finite element-based analysis of single-crystal Si contour-mode electromechanical RF resonators” 2004/8/MEMS, NANO and Smart Systems, 2004. ICMEMS 2004. Proceedings. 2004 International Conference on 25-27, pp. 414-418, August 2004
[Non-Patent Document 14]
“Stemless wine-glass-mode disk micromechanical resonators” 2003/1/Micro Electro Mechanical Systems, 2003. MEMS-03 Kyoto. IEEE The Sixteenth Annual International Conference on 19-23 Jan. 2003, pp. 698-701, January 2003
[Non-Patent Document 15]
“1.14-GHz self-aligned vibrating micromechanical disk resonator” 2003/6/Radio Frequency Integrated Circuits (RFIC) Symposium, 2003 IEEE, pp. 335-338, June 2003
[Non-Patent Document 16]
“A sub-micron capacitive gap process for multiple-metal-electrode lateral micromechanical resonators” 2001/1/Micro Electro Mechanical Systems, 2001. MEMS 2001. The 14th IEEE International Conference on 21-25 Jan. 2001, pp. 349-352, January 2001
A current technological level will next be described in the following.
As for problems of a filter system using external parts (for example, an external coil), a filter characteristic that selects only a desired channel is desired from a need of reception performance.
However, there is a limitation to characteristics of theoretical frequency selectivity of a filter formed by a resonant circuit in which a coil and a capacitor are connected to each other, and high-image-quality reception is restricted by an effect of noise signals of adjacent channels.
On the other hand, as described above, the filter system using the resonant circuit of the coil and the capacitor causes the following problems when the value of the coil is not optimized for each frequency region. For example, because the pass band width of the channel filter varies greatly, a large number of coil parts are used, which is a great impediment to reduction of manufacturing cost.
In addition, the techniques of forming an on-chip filter on a current wafer have not achieved a level of filter characteristics for realizing a television channel filter having excellent selectivity.
In order to realize an excellent television channel filter as a wafer on-chip filter economically reasonably, a large number of resonator elements of different frequencies need to be simultaneously formed en bloc. For 60 channels, for example, at least 120 kinds of resonator elements of different frequencies are necessary.
Thus, it is clear that the adoption of a resonance mode in a lateral direction (a system of designing resonance frequency by dimensions of a mask, and typically area vibration/shear vibration/elongation vibration) is very useful (see FIG. 26).
However, at present, in the lateral resonance mode, a resonator element in the most advanced stage of development has not achieved a lateral electromechanical coupling factor (k2) of 1% or more.
It is considered that the electromechanical coupling factor (k2) of a MEMS resonator element not using a piezoelectric material has a limit of about 0.5%.
Moreover, filters in these lateral direction resonance modes have not achieved even a fractional bandwidth of 1.70% (see Non-Patent Documents 1 to 16, for example).
Technical problems that cannot be overcome by the related techniques alone will be described in the following.
The following problems are present before a filter for TV channels is realized by a band-pass filter formed by piezoelectric resonator elements on a wafer by a semiconductor manufacturing process.
No provision has been made for a wide frequency range (from the VHF band to the UHF band). In addition, in a band-pass filter capable of channel selection with a fixed bandwidth, a practical method such as simultaneously forming resonator elements intended for the whole of a wide range of fractional bandwidth, which changes with variation in the frequency region, en bloc on a single chip on a wafer has not been proposed nor realized.
For example, there is a desire fo a band-pass filter capable of channel selection with bandwidth fixed at 6 MHz, 7 MHz, and 8 MHz, for example, as fixed bandwidth, as shown in Table 1.
In this band-pass filter, the fractional bandwidth (%) which changes with variation in the frequency region is 11.1% to 0.74% in terrestrial digital broadcasting all over the world, for example. In order to achieve the whole of such a wide range of the fractional bandwidth, the electromechanical coupling factor of resonator elements needs to be 9.09% to 0.60%.
However, a practical method such as simultaneously forming such resonator elements en bloc on a single chip on a wafer has not been realized. It is possible, however, to achieve only specific fractional bandwidths.
TABLE 1FRACTIONAL BAND WIDTHS (%) OF TV CHANNELS AND ELECTROMECHANICAL COUPLING FACTORSREQUIRED OF CORRESPONDING RESONATOR ELEMENTSVHF (L-BAND) 5chVHF (H-BAND) 11chUHF 49ch to 56ch54 MHz72 MHz76 MHz88 MHz174 MHz216 MHz244 MHz470 MHz806 MHz862 MHzFRACTIONALBAND WIDTH (%)OF TV CHANNELBANDJAPAN6 MHz1.28%0.74%NORTH6 MHz11.11%8.33%7.89%6.82%3.45%2.78%1.28%0.74%AMERICAEUROPE7 MHz4.02%2.87%8 MHz1.70%0.93%ELECTRO-MECHANICALCOUPLINGFACTOR (%)REQUIRED OFRESONATORELEMENTJAPAN6 MHz1.02%0.60%NORTH6 MHz9.09%6.78%6.42%5.53%2.78%2.23%1.02%0.60%AMERICAEUROPE7 MHz3.24%2.31%8 MHz1.37%0.74%
In each frequency region of the UHF band, a VHF high-frequency band (hereinafter referred to as VHF-H), and a VHF low-frequency band (hereinafter referred to as VHF-L), resonance modes for different electromechanical coupling factors are selected, and filter design is made. Design has not been made which then realizes a filter in which consecutive frequency channels are sequentially arranged along frequency within each of the frequency regions (UHF, VHF-H, and VHF-L) and which filter handles the consecutive variations in fractional bandwidth.
Generally, in a filter constitution based on electric coupling of a resonator element formed on many wafers thus far, one resonator element forming a filter is formed entirely by one vibrator. This is because the design of the vibrator coincides with the design of the resonator element, and thus filter design can be greatly simplified and made easily. On the other hand, however, there is a very difficult problem in impedance matching (50Ω to 150Ω), which consequently invites a great loss of a pass region of the filter.
When television channels and the like are realized by a band-pass filter by arranging all channels on a wafer chip, a yield tends to be decreased due to variations in precision of resonator elements in manufacturing processes. In addition, in order to make provision for digital television broadcasting all over the world, resonator elements need to be made differently according to specifications of different countries. Therefore, manufacturing cost is increased, and manufacturing becomes complex.
In addition, as in ordinary MEMS (Micro Electro Mechanical Systems) devices, the hollow structure has invited an increase in cost of forming a package.